The present invention relates to a brake-pressure control device for a road vehicle having a hydraulic multi-circuit braking system, and, more particularly, to a brake-pressure control device in which there is provided for generating the brake pressures to be fed into the wheel brakes a master cylinder which can be actuated by means of a pedal-controlled pneumatic brake booster provided with a change-over device which responds to the actuating behavior of the driver and which causes an automatic change-over of the brake booster from a value of the boost factor required for limited braking to a higher value of the boost factor required for full braking, at least in the event that the speed .phi. with which the driver actuates the brake pedal in the introductory phase of a braking operation exceeds a threshold value.
Generally speaking, a brake-pressure control device is provided in a brake system in which a master cylinder actuatable by a pedal-controlled pneumatic brake booster generates the brake pressures to be fed into the wheel brakes. The control device includes a change-over device which responds to the actuating behavior of the driver and brings about an automatic change-over of the brake booster from a value of the boost factor required for limited braking to a higher value of the boost factor required for full braking, at least in the event that the speed .phi..sub.s with which the driver actuates the brake pedal in the introductory phase of a braking operation exceeds a threshold value .phi..sub.s. A brake-pressure control device of this type is the subject of the applicant's earlier German unpublished patent application P 42 08 496.2.
In the aforementioned brake-pressure control device, the vacuum brake booster is provided with a solenoid-valve arrangement which, by actuation with an electrical output signal from the electronic control unit, can be driven out of the basic position, in which a control channel of the control part of the brake booster, via which pressure compensation can take place between the vacuum chamber and the driving chamber of the brake booster, is connected to communicate with the vacuum chamber; the driving chamber is shut off from the outside atmosphere, into an energized operating position in which the driving chamber of the brake booster is loaded with the ambient pressure via a throughflow path opened in this operating position and belonging to the solenoid-valve arrangement, but the control channel is shut off from the vacuum chamber.
The basic position of the solenoid-valve arrangement is assigned to the non-actuated state of the brake system and its limited-braking mode. The valve arrangement is changed over into its energized position when, during the actuation of the brake pedal, the threshold value .phi..sub.s of the actuating speed is exceeded. The control channel of the control part is guided outwards via a flexible line and a bushing of the housing of the vacuum chamber. The solenoid valve is arranged in the outer space of the vacuum chamber and, in its basic position, connects this bushing to a second bushing leading back into the vacuum chamber. In the energized operating position of the solenoid valve, which is assumed when the valve is actuated by an output signal characteristic of the exceeding of the threshold value .phi..sub.s of the actuating speed of the brake pedal, the first bushing is shut off from the second bushing and the latter is instead connected to the outside atmosphere, so that the brake booster works with maximum brake boosting.
The brake-pressure control device explained thus far has the disadvantage in functional terms that, when the change-over device responds, the increase in brake force takes place, as it were, abruptly. For a minimum response time, it is virtually impossible for the driver to influence the brake force, and this can considerably impair at least the driving comfort during a braking operation. A constructional disadvantage is that the solenoid valve arranged outside the housing of the brake booster requires considerable space which is exceedingly difficult to provide in the conventional arrangement of the booster/master-cylinder unit in the upper rear part of the engine space, where the hydraulic unit of an anti-lock system, which necessary in combination with the brake-pressure control device, is also often accommodated.
An object of the present invention is, therefore, to improve a brake-pressure control device to the effect that, while having an improved response behavior in the sense of better metering of the brake force or vehicle deceleration, it can nevertheless be produced with a small space requirement.
This object has been achieved, according to the present invention, by providing that the reaction piston is of two-part design and comprises a first part-piston which can be supported axially on the reaction disc and which is guided axially displaceably, but non-rotatably, on the housing part carrying the reaction disc, as well as a second part-piston which forms the sealing flange of the inlet valve and which is rotatable, the two part-pistons being in engagement with one another via complementary threads extending in the axial direction; the change-over device comprises an electric motor of reversible direction of rotation, by means of which the second part-piston can be driven in rotation via a reduction gear, while, in the event that the driver actuates the brake pedal at high speed at the start of a braking operation, an activation of the electric motor takes place in that direction of rotation which ensures that the relative rotational movement of the two part-pistons leads to a shortening of the reaction piston as a whole and, at the latest at the end of a braking operation, an activation of the electric motor takes place in the opposite direction of rotation, until a position of the part-pistons of the reaction piston corresponding to a maximum length of the latter is reached again; the electric motor is arranged in a pedal-side end portion of the control-part housing, so as also to execute the movements of the latter, and is coupled in drive terms to the rotatable part-piston via a hollow shaft coaxially surrounding the pedal tappet.
Accordingly, a two-part construction is provided for the reaction piston, with a first piston element which can be supported axially on the reaction disc and which is guided axially displaceably, but non-rotatably, on the housing part carrying the reaction disc, and with a second piston element which forms the sealing flange of the inlet valve and which is rotatable. The piston elements of the reaction piston are in engagement with one another via complementary threads extending in the axial direction. The change-over device comprises an electric motor of reversible direction of rotation, by way of which the second piston element forming a sealing flange can be driven rotationally via a reduction gear, while, in the event that the driver actuates the brake pedal at high speed (.phi.&gt;.phi..sub.s) at the start of a braking operation, an activation of the electric motor takes place in that rotation direction which ensures that the relative rotational movement of the two piston elements leads to a shortening of the reaction piston as a unit. At the latest at the end of a braking operation, an activation of the electric motor takes place in the opposite rotation direction, until a basic position, corresponding to a maximum length of the reaction piston, of its piston elements is again reached. The electric motor is arranged in a pedal-side end portion of the control-part housing so as also to execute the movements of the latter and is drivingly coupled to the rotatable reaction-piston element via a hollow shaft coaxially surrounding the pedal tappet.
An advantage of the brake-pressure control device according to the present invention is that, when the change-over device responds, the brake force does not abruptly assume a high value, but increases continuously with a time behavior predetermined by the speed of the electric motor. The pedal reaction detectable by the driver is essentially that the brake pedal can be actuated with little force until the brake booster finally reaches the drive level, that its driving chamber is under atmospheric pressure and a further increase in brake pressure, without boosting, takes place only in correlation with an increase in the force with which the driver actuates the brake pedal. Since the forces necessary for the opening actuation of the inlet valve of the brake-pressure control device according to the present invention are low, the control motor can be a motor with a power consumption of a few watts, i.e. of sufficiently small construction to ensure that it can be installed in the pedal-side end portion of the control-part housing with its motor shaft parallel to the central axis of the control part. The motor drives, via a pinion of small diameter, a gearwheel which is connected rotationally fixedly to the hollow shaft and the diameter of which is between three (3) and eight (8) times larger than that of the pinion in order to ensure a sufficient reduction of the motor speed.
Both in reigns of as space-saving a form of construction as possible and from the point of view of an exact mounting of the hollow shaft provided for the opening control of the inlet valve, it is especially advantageous if the hollow shaft is mounted slidably on a cylindrical portion of the pedal tappet passing centrally through the control-part housing, which achieves a large "bearing length", within which transverse forces acting on the reduction gearwheel connected to the hollow shaft can be absorbed effectively.
If, as in the preferred configuration of the brake-pressure control device, the reaction-piston element engaging on the reaction disc is a threaded cap whose thread is in engagement over a portion of its length with an end portion, configured as a threaded bolt, of the rotatable reaction-piston element, and, in combination therewith, the reaction-piston element supportable on the reaction disc can be supported axially on a stop element arranged fixedly relative to the control-part housing, after a small initial portion of the relative stroke of the two reaction-piston elements has taken place in the event of an activation of the brake-pressure control device, then the brake-pressure control device can also be utilized for an automatic actuation of the brake system. That is, an actuation takes place without the cooperation of the driver, for example for the purpose headway control and/or drive-slip control and/or for an activation, desirable for other reasons relating to driving dynamics, of one or more wheel brakes of the vehicle.
In one particularly advantageous embodiment of the brake-pressure control device of the present invention, the valve spring, which urges the annular disc valve element of the control part of the brake booster into bearing contact with the seat of compensating valve and with the reaction-piston element of the inlet valve and which, in a conventional brake booster, is supported on an annular shoulder of the pedal tappet, is supported on a radially inward-projecting supporting flange of a flanged sleeve. The latter is inserted firmly into the control-part housing, and the rotatable hollow shaft which prevents the valve spring from being supported on the pedal tappet, passes through the flanged sleeve.
The objects of the present invention are also achieved in that the housing-side valve element of the compensating valve, on which the valve element in the annular disc form can be supported by its radially outer region, comprises a sleeve-shaped part which is guided on the control-part housing displaceably in a pressure-tight manner in the axial direction. The sleeve-shaped part is in engagement, via a thread extending in the axial direction over a length portion of the sleeve casing, with a complementary thread of a rotatably mounted hollow shaft which, in the tubular portion of the control-part housing through which the pedal tappet passes centrally, coaxially surrounds the latter at a radial distance and is sealed off from the tubular portion of the control housing and which can be driven in rotation by an electric motor of reversible rotational direction, provided as an actuating device of the change-over device, but is secured against axial displacement within the control-part housing, while, in the event of an actuation of the brake pedal taking place at excessive speed (.phi.&gt;.phi..sub.s), the electric motor is activated in that rotation direction in which the axial displacement, resulting from the rotation of the hollow shaft, of the sleeve-shaped compensating-valve element, on which is supported the annular-disc shaped valve element, which is also a valve element of the inlet valve, leads to a lifting off of the annular-disc shaped valve element from the sealing face of the reaction piston. At the latest at the end of the brake actuation the electric motor is driven in that rotation direction in which the axial displacement, resulting therefrom, of the sleeve-shaped compensating-valve element once again returns into the basic position of the annular-disc shaped valve element. The basic position corresponds to the blocking state of the inlet valve.
The immediately-above-described embodiment of the brake-pressure control device provides, both an initiation of a braking operation with a very rapid rise in brake pressure and an automatic actuation of the brake system, as is necessary, for example, for a headway-control mode or a drive-slip control mode.
If the electric motor, which can once again be rated at an advantageously low power consumption, is accommodated in a pedal-side end portion of the control-part housing and the reduction ratio of the gear formed by a driving pinion of the motor and a driving toothing of the hollow shaft is between 1/4 and 1/8, then it is especially advantageous if the toothing with which the driving pinion of the electric motor meshes is configured as an internal toothing arranged on the pedal-side end portion of the hollow shaft. This allows an especially space-saving arrangement of the electric drive motor which can then be accommodated for the most part within the clear cross-section of the tubular portion of the control-part housing.
The necessary securing of the axially displaceable valve element of the compensating valve can be achieved in a simple way by a small tube which forms part of the passage channel connecting the vacuum chamber to the annular space of the control part communicating with the vacuum chamber and which starts from the valve element and is displaceable in a pressure-tight manner in a housing bore otherwise limiting the channel.
In order to hold the annular-disc shaped valve element in the form of an annular disc securely in the position necessary for the various valve functions, the valve spring urging this valve element into bearing contact with the axially movable rib-shaped valve element of the compensating valve of the control part is supported on an inner radial step of a flanged sleeve which coaxially surrounds the pedal tappet at a clear radial distance. The flanged sleeve also has an outer radial step, on which the sealing cuff is supported axially and, via a casing-shaped portion of the flanged sleeve connecting the two steps to one another, radially, and is held in sealing bearing contact with the inner cylindrical surface of the rotatable hollow shaft. The flanged sleeve is arranged rotationally fixedly within the control-part housing.
The relevant securing of the flanged sleeve against rotation can be achieved in a simple way in that the flanged sleeve is provided, within a casing portion pointing towards the sealing element in the form of an annular disc, with a transverse bolt passing through a long hole of the pedal tappet which is itself non-rotatable as a result of connection to the brake pedal.